a) Field of the Invention
The present invention relates to a split polyethylene stretched material and a process for producing said material. More particularly, the present invention relates to an ultra-high-molecular-weight polyethylene stretched material which has a high strength and flexibility and which can be easily laminated to other materials.
b) Description of the Related Art
It is known to mold an ultra-high-molecular-weight polyethylene into a sheet or a tape and then stretch the sheet or tape to obtain a high-strength and high-modulus polyethylene material.
For example, it is described in Japanese Patent Application Laid-Open No. 130313/1984 to melt-mix an ultra-high-molecular-weight polyethylene and a wax, extrude the resulting mixture, and cool, solidify and then stretch the extrudate.
Also, it is described in Japanese Patent Application Laid-Open No. 101032/1985 to cool an ultra-high-molecular-weight polyethylene solution, compression-mold the resulting gel-like material, and stretch the compression-molded product.
Further, it is described in EP 253513A1 (1988) to compression-mold an ultra-high-molecular-weight polyethylene at a temperature lower than the melting point of the polyethylene and stretch the compression-molded product.
The ultra-high-molecular-weight polyethylene sheets or tapes obtained by the above processes are lightweight and have a high strength and excellent water resistance; therefore, they are intertwisted to be used as marine ropes, or weaved into cloths to be used as explosion-proof sheets, etc.
These ropes or cloths, however, have not only a high-strength but also a high modulus; therefore, they lack flexibility. Moreover, the above stretched sheets or tapes, when laminated to obtain a laminate or made into a composite material with other material, are unable to produce a sufficient adhesion strength because their area of contact with a resin or other material is limited.
Conventional split yarns are produced mainly from a polyolefin (raw material). They are used in ropes for ships, yachts, mount climbing, etc.; threads such as machine cotton, thread for making tatami, and the like; yarns for bag making; and so forth. Also, webs obtained by subjecting a wide stretched film to splitting are used as non-woven fabrics obtained by laminating a split fiber warp sheet and a split fiber weft sheet, or cross-laminated non-woven split fabrics.
Split yarns have various features; for example, they are softer than conventional monofilaments and have a square cross-section, a large surface area and a high friction coefficient. They further have a feature in that they can be made into a mono-axially stretched wide reticulate web. The biggest feature of split yarns, however, is that they can be produced at a low cost by using a simple apparatus which comprises only a flat yarn-producing apparatus and a splitter integrated thereinto. Because of the low cost, split yarns are used in industry.
Conventional split yarns ordinarily have a strength of about 3-4.5 g/d (a strength per denier expressed in g) which is not so high. Therefore, if their strength can be increased 2- or 3-fold with their low cost feature being maintained, such an improved split yarn can be used for the manufacture of, for example, a very light rope for mountain climbing a light and strong flexible container and a light and strong cross-laminated non-woven fabric. The improved split yarn can also find new applications such as polyolefin prepreg and the like, by further utilizing the other properties, i.e. large surface area and spreadability to crosswide direction. Thus, it has been strongly desired to develop a low cost and strong split yarn; however, none of the conventional techniques have been able to provide such a split yarn.